1. Field of the Invention
This invention relates to global positioning system based navigation techniques for developing position and velocity data required for autonomous operation of satellites incorporating on-board, event-based command systems.
2. Discussion of the Related Art
Spacecraft tracking and navigation began when The Johns Hopkins University Applied Physics Laboratory determined Sputnik's orbital parameters as a function of the doppler shift of an on-board radio beacon. That was in 1957. Soon thereafter, that organization devised and developed the Navy Transit Satellite Navigation System which provided global navigation services for the military and civilian community for over 35 years. The “TRANSIT” system used a network of low earth orbit satellites and therefore it was unacceptable for most spacecraft navigation applications.
A limited number of experimental systems developed by the Applied Physics Laboratory (APL), NASA and the U.S. Air Force provided semi-autonomous means for orbit determination and prediction. The experimental system provided ground-based orbit determination for spacecraft through the use of complex ground stations and coherent transponders that reside on the spacecraft to be tracked. The French National Space Agency has operated a similar system, DORIS, since the early 1990s. However, the high cost of operating such systems has forced spacecraft operators to find other tracking methods.
Over the last three decades, APL and other organizations have demonstrated the feasibility of using GPS for positioning satellites and other high dynamic platforms. For over 20 years, the APL developed SATRACK system has utilized GPS translators and ground-based signal processing systems for trajectory reconstruction and guidance system evaluation of Navy Trident missiles. The first GPS-based navigation of a satellite occurred on Transat, an APL spacecraft launched in 1978 and operated for over 10 years. Transat was a Transit navigation satellite with an on-board GPS translator.
Autonomous positioning of satellites using space borne GPS receivers was first demonstrated in the early 1980s when APL developed and flew four GPSPAC systems, and in the early to mid 1990s when NASA and the Jet Propulsion Laboratory used GPS receivers on the TOPEX/Poseidon spacecraft. More recently, other programs have adopted the use of GPS-based navigation systems for spacecraft. For instance, U.S. Pat. No. 5,109,346 for “Autonomous Spacecraft Navigation System” issued to J. Wertz on Apr. 28, 1992 describes a system using onboard observations of the earth, sun and moon to determine spacecraft attitude, instantaneous position and orbit based on multiple position estimates. Position and orbit data are derived by multiple deterministic solutions, including some that employ star sensors and gyros, and the multiple solutions are accumulated in a Kalman filter to provide continuous estimates of position and orbit for use when the sun or moon is not visible.
Developing totally autonomous satellites results in large cost savings because of the elimination of ground support stations. In addition, ground supported satellites are subject to ongoing maintenance costs and are prone to serious malfunction if ground support stations are damaged or if control signal transmissions are interfered with.